Wave energy converters utilizing pressure differences

ABSTRACT

An elongated cylinder is fully submerged, in vertical orientation, just below the mean water level of, e.g., and ocean, and of a length, dependent upon surface waves of preselected wavelength, such that the top of the cylinder experiences relatively large pressure variations in response to over passing waves while the bottom of the cylinder experiences an almost steady pressure substantially independent of the over passing waves. The pressure differential over the length of the cylinder is used for causing relative movements between the cylinder and adjoining water, and such relative movements are used for driving a piston of an energy converter. The cylinder can be hollow and in fixed location for causing water movements through the cylinder, or the cylinder can move through the water relative to a fixed transducer. In one version of the movable cylinder, the transducer is fixedly mounted on a fixed in place float disposed within the movable cylinder. In a second version, the transducer is fixedly mounted beneath the movable cylinder on the ocean floor, and the cylinder is coupled to the transducer.

This is a division of U.S. patent application Ser. No. 09/763,247, filedOct. 15, 2001, now U.S. Pat. No. 6,768,216 which application claims thebenefit of the filing date of international application PCT/US00/14652,filed 26 May 2000 which claims the benefit of Povisional application No.60/136,189, filed May 27, 1999.

BACKGROUND OF THE INVENTION

This invention relates to the conversion of energy from naturallyoccurring sources of mechanical energy, and particularly to theconversion of the mechanical energy present in ocean surface waves touseful energy, particularly electrical energy.

In many known systems for capturing surface wave energy, a float is usedfor being vertically oscillated in response to passing waves. The floatis rigidly coupled to an energy converter which is driven in response tovertical movements of the float. In one system, described in U.S. Pat.Nos. 4,773,221 and 4,277,690 (the subject matter of which isincorporated herein by reference), an open-ended hollow tube is rigidlysuspended beneath a float, the tube being completely submerged and invertical orientation.

The tube vertically oscillates in the water in correspondence withmovements of the float and, in the absence of anything within the tube,the tube moves freely relative to the column of water within theopen-ended tube. In one embodiment, a movable piston is disposed withinthe tube for blocking relative movements between the water column andthe tube. As the tube and float oscillate within the water, the mass ofwater within the tube tends to block corresponding movements of thepiston, hence the piston moves relative to the tube. Actual movements ofthe piston do occur, however, and provided the entire system isoscillating at its natural resonant frequency, relatively largeamplitude oscillations of the piston can occur. The moving piston drivesan energy converter fixedly mounted, e.g., within the float, forconverting the piston movements to useful energy.

While these float driven tube systems work, efficient operation requiresthat the natural resonant frequency of the system closely matches thefrequency of the ocean waves driving the system. While this can begenerally accomplished at a specific site and specific time,particularly if means for adjusting the resonant frequency of the systemin response to changing surface wave frequencies are provided, a problemis that, at any instant, multiple random surface waves are presentwhereby much of the wave energy present can not be efficientlytransferred to the oscillating system. Also, the means for adjusting theresonant frequency of the device generally involves changing the watermass within the device. Since this mass is quite large, it is notreadily changed.

A feature of the present invention is that a relatively high efficiencyof operation is obtained which is relatively insensitive to randomvariations of wave frequencies and amplitudes.

SUMMARY OF THE INVENTION

In a first embodiment of the invention, an open-ended, hollow tube isdisposed in vertical, submerged and fixed location relative to the meanwater level. Specifically, the tube is not in “floating” (moveable)relationship with the passing waves. The length of the tube and thedepth of the top end of the tube beneath the mean water level areselected, as described hereinafter, depending upon the frequency andamplitude of the most prevalent anticipated surface waves, as well asthe water depth. While maximum efficiency of operation is attained whenthe anticipated waves are present, the fall-off of efficiency ofoperation is relatively small with variations of wave conditions.

During operation, pressure variations, at the top, open end of the tube(caused by passing waves) in comparison with a relatively fixed pressureat the open, bottom end of the tube (unaffected by passing waves) causevertical flows of water through the tube which are used for driving anenergy converter, preferably by means of a movable piston within thetube.

In a second embodiment of the invention, a hollow tube having a closedtop end and a bottom open end is disposed in vertical, submerged butrelatively movable relation with the mean water level. In a preferredembodiment, the tube is secured for vertical cyclical movements relativeto a float fixedly submerged beneath the water surface and disposedwithin the tube. The dimensions of the tube and its at-rest locationrelative to the water surface are in accordance with the tube of thefirst embodiment except, in the second embodiment, the piston movablewithin the tube of the first embodiment comprises the closed top end ofthe tube of the second embodiment. During operation, pressure variationsbetween the top and bottom ends of the tube cause vertical oscillationsof the tube relative to the fixed float, and such oscillations are usedfor driving an energy converter.

In all embodiments, movements of the tube relative to the adjoiningwater are caused not by wave-induced displacements of a float on thewater surface, but in response to pressure variations caused by passingwaves. Also, as noted hereinafter, tilting movements of the float causedby water movements can be used for capturing wave energy.

DESCRIPTION OF THE DRAWINGS

The drawings are schematic and not necessarily to scale.

FIG. 1 is a sketch for identifying various relevant dimensionalparameters of a system according to the present invention deployed in abody of water;

FIGS. 2, 2A and 3-6 are side sectional views showing differentembodiments of power converting systems in accordance with a firstembodiment of the present invention deployed in bodies of water, e.g.,an ocean;

FIG. 7 is a side elevational view of an energy converter in accordancewith a second embodiment of the invention;

FIG. 8 is an end view of the converter looking in the direction of thearrows 8—8 in FIG. 8;

FIG. 9 is an isometric view of the converter shown in FIGS. 7 and 8; and

FIG. 10 is a side elevational view of a modified version of theembodiment shown in FIGS. 7-9.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

An apparatus according to a first embodiment of the present invention isshown in FIG. 1. Shown schematically is an open-ended tube 10 disposed(as herewith described) in fixed, vertical orientation below the meanwater level of a body of water, e.g., an ocean having wind drivensurface waves. FIG. 1 also identifies parameters important in thepractice of the invention, i.e., wave height, wave length, water depth,depth of the top of the tube below the water surface, length of thetube, and the diameter of the tube. The optimum depth for the lower endof the tube is dependent on the wavelength (λ) of the longest waves tobe utilized in an efficient manner. The principle of operation is thatthe changes in water energy level, which can be expressed as changes inpressure, due to the passage of wave peaks and troughs, is highest nearthe surface, and these pressure changes decay exponentially with depthbelow the surface. Thus, the top of a long tube experiences relativelylarge pressure variations while the bottom of the tube experiences analmost steady pressure that is equal to the pressure due to the weightof water above it at the mean water level.

The energy levels at different water depths under a wave field can becalculated with Equations 1 and 2. The equations are for deep waterwaves and are modified somewhat by the depth in more shallow water(depths less than λ/2). The water energy levels due to waves of a givensize are a function of wave length and water depth. There is littlepractical value in extending the tube bottom any deeper than ½ thewavelength of the longest waves to be optimally used because the energylevel is already greatly reduced from its near surface value.E _(d) =E _(s)exp (−2πd/λ)  (1)where: E_(s) is the energy due to a wave at the water surface, and

-   -   E_(d) is the energy due to a wave at a depth equal to d, and    -   λ is the wave length of the waves being considered        The wave length of deep water waves may be calculated by the        formula:        λ=gT ²/2π  (2)        where: g is the gravitational constant, 9.8 meters per second        per second, and    -   T is the period of the waves in seconds        As an example using equation 2, for waves with a period of 7        seconds, the wave length is    -   λ₇=76.43 meters        As a second example, for waves with a period of 5 seconds, the        wave length is    -   λ₅=38.99 meters        For waves with a period of 7 seconds and a wave length of 76.43        meters, and waves with a period of 5 seconds and a wave length        of 38.99 meters, the energy at different depths can be        calculated as percentage of the energy at the surface, using        Equation 1. This is shown in Table 1.

TABLE 1 energy water depth as a % E_(d7) as depth as a % E_(d5) as depth(m) of λ₇ % of E_(s) of λ₅ % of E_(s) 0.1 0.1 99.2 0.3 98.4 0.5 0.6 96.01.3 92.3 1.0 1.3 92.1 2.6 85.1 19.1 25 20.8 49 4.6 38.21 50 4.3 98 0.276.43 100 0.2 196 0Table 1 shows that when waves with a period of 7 seconds are present,and the tube 10 has its top end at depth of 0.5 meters below thesurface, and its bottom end at a depth of 38.21 meters below thesurface, the top will experience pressure changes 91.7% (96−4.3) largerthan the bottom. These conditions will cause water to flow down theinside of the tube when a wave peak is over the top end, and water toflow up the inside of the tube when a wave trough is at the top of thetube. This pressurized water flow provides the opportunity to extractmechanical power from the wave energy. Extending the tube from 38.21meters to 76.43 meters in length only increases the pressuredifferential by 4.1% (4.3−0.2).

Further study of the Table 1 shows that when waves with a 5 secondperiod are present, a tube with its bottom at 38.21 meters below thesurface has an even lower pressure variation at the bottom, 0.2%, thanwhen 7 second period waves are present. Thus wave energy from shorterwave length and shorter period waves can be collected efficiently. Whenwaves of longer period are present, the energy, or pressure variations,at the tube bottom gradually increase. Thus, the efficiency of energycollection will gradually decrease. However, the range of efficientoperation is much larger than in the previously described known devicesthat are tuned for specific wave periods for resonant and efficientoperation. These devices can suffer a significant loss of efficiencywhen the wave period changes even a few seconds.

Table 1 also shows that as the wave period decreases, the importance ofthe top of the tube being near the surface increases. For example, witha water depth of the top of the tube being 0.5 meters under the surfacein 7 second period waves, the energy has decreased at the tube top to96% of its maximum, while in 5 second waves the energy has decreased to92.3% of its maximum.

Regular waves are waves that have a consistent period. A sine wave is anexample of a regular wave. Regular waves at a constant period wouldallow the tuning of a resonant wave energy capture device to thespecific wave period, even though the wave period may change with thenegative impact mentioned above. In practice, ocean and sea waves areboth random and irregular and simultaneously contain waves withdifferent periods. An example of this is a case when ocean swells with a10 second period are present along with wind waves with a 5 secondperiod. The inventive apparatus has the ability to capture energyefficiently from irregular waves as well as from regular waves. This isbecause the apparatus is not optimized for a specific period but isdriven dependent upon the instantaneous quantity of water above or belowthe mean water level (but subject to “cancellation effects” discussedhereinafter).

The theoretical amount of energy that can be captured at a site with agiven water depth and wave characteristics can be determined as follows.Bernoulli's Equation for fluids in unsteady irrotational flow is:${\frac{\delta\quad\phi_{1}}{\delta\quad t} + \frac{P_{1}}{\rho} + {g\quad y_{1}} + \frac{V_{1}^{2}}{2}} = {\frac{\delta\quad\phi_{2}}{\delta\quad t} + \frac{P_{2}}{\rho} + {g\quad y_{2}} + \frac{V_{2}^{2}}{2}}$Where (1) is a point in the fluid and (2) is another point in the fluid,and where:

-   -   δφ/δt is the differential of the velocity potential in meters        squared per second squared (m²/s²), at a point, and    -   gy is the gravitational constant times the depth at a point in        m²/s², and    -   P/ρ is the pressure at a point divided by the fluid density in        m²/s², (to achieve these dimensions it should be remembered that        mass can be expressed as force divided by acceleration), and    -   V²/2 is the velocity squared of the fluid at that point m²/s².        For example, if point 1 is considered to be near the water        surface as is the top of the tube, and point 2 is deeper as is        the bottom of the tube, the Bernoulli Equation can be the basis        for analysing the different forms of energy available at each        point as time passes.        Principle of Operation—Energy Capture: The simple long tube        described above provides a situation where there are two        different pressure levels appearing simultaneously at each end        of the tube. Two preferred methods of capturing energy from the        available energy in the tube are as follows:

-   1. A piston 12, shown in FIG. 2, placed in the tube is forcefully    driven up and down by water in the tube moving up and down due to    the varying pressure differentials above and below the piston. This    forceful movement is converted to mechanical power by attaching a    device to the piston that resists its movement. One example is the    rod of a hydraulic cylinder. The motion of the cylinder rod pumps a    pressurized fluid (hydraulic fluid) through a hydraulic motor which    then rotates. The mechanical power produced by the hydraulic motor    is converted to electrical power by a generator attached to the    motor. In FIG. 2, the water driven piston 12 and its shaft are shown    to move up and down while guided by the Piston Shaft Support and    Shaft Bearings 16. To reduce mechanical drag on the system the    piston preferably does not touch the sides of the tube. A clearance    between the piston rim and tube of 3 to 6 millimeters will permit    some water to leak past the piston. This represents a loss of power    but is a small percentage of the area for a piston that is larger    than 1 meter in diameter. A hydraulic cylinder rod 18 (from a    hydraulic cylinder 20) is attached to the top of the piston shaft    12. A hydraulic cylinder support 22 fixedly attaches the cylinder 20    to the tube 10. Hydraulic hoses 24 carry the hydraulic fluid back    and forth to a watertight compartment that contains an hydraulic    motor and electric generator. A double-ended cylinder (rod extends    from both ends) is preferred because the cylinder performance is the    same in both stroke directions. The piston is made buoyant enough to    cause the piston—piston shaft—hydraulic cylinder rod assembly to be    neutrally buoyant, and therefore move up or down equally with the    same applied forces. A preferred arrangement of the components is    shown in FIG. 2A. In this arrangement, the piston 12 slides up and    down on the hydraulic cylinder itself. Both ends of the hydraulic    cylinder rod 18 are fixedly attached to the tube 10 by the hydraulic    cylinder rod supports 22. A watertight compartment 26 is part of the    piston assembly and contains the hydraulic motor and electric    generator. This compartment is buoyant enough to cause the entire    piston assembly to be neutrally buoyant.    -   FIG. 2 also shows an arrangement for mooring the power        converting system. This is later described.    -   FIG. 3 shows an arrangement where the area of the piston 12 is        larger than the area of the tube 10 at its top and bottom ends.        This is to illustrate that the piston area can be either larger        or smaller than the tube end areas. In a given situation, the        arrangement in FIG. 3 will produce a higher force and a shorter        stroke than if the piston and tube ends have the same area. This        is because the tube length and depth determines the pressure        differential on the piston, and the tube end areas determine the        volume of water flow. Thus, the same pressure on a larger piston        area produces more force, but more water volume is required to        move the larger piston. FIG. 3 illustrates that the piston size        can be varied to match desired piston forces and strokes.        However there are losses of energy incurred whenever the moving        water is caused to change direction as it does when the piston        area is different than the tube end areas. Thus, the most energy        efficient configuration is when the piston and tube ends have        the same area.    -   A second power take-off approach (not illustrated) is to attach        a rod to the piston that moves vertically with the piston.        Instead of this piston rod being attached to a hydraulic        cylinder, it is attached to a positive drive belt (the belt and        sprockets having teeth that are positively engaged), that is        around two vertically arranged sprockets. As the piston is        driven up and down by the wave energy it drives one side of the        belt up and down causing the sprockets to rotate. One of the        shafts of a driven sprocket is coupled to a generator to produce        electric power.    -   A third power take-off approach (not illustrated) is to directly        drive a linear generating device, such as a linear electric        motor, with the piston movement. Due to the sub-surface marine        environment, the hydraulic approach is preferred.

-   2. In the system shown in FIG. 4, a turbine 40 is disposed within a    tube 10 for being driven to rotate by the water flow in the tube    moving up and down due to the varying pressure differential at the    top and bottom of the tube. This rotation produces mechanical power    by, for example coupling the shaft of an electric generator 42 to    the turbine shaft 44. The tube preferably has, as shown, large    diameter ends, and a small diameter turbine section in order to    increase the water flow velocity through the turbine.    -   The moving piston approach (1) is preferred because in general,        the inventive systems are more readily designed for providing        powerful strokes of limited length rather than providing rapid        water flow. Each approach is described in more detail below:        1. Moving piston approach: As a piston such as shown as 12 in        FIGS. 2, 2A, and 3 moves against resistance it produces a force        (Newtons). The piston moves a certain distance in a given time        (meters per second). The product of this force times velocity is        Newton-meters per second (Nm/s) which converts directly to watts        of power. One Nm/s is equal to one watt.        Power_(watts)=Force_(Newtons)×Velocity_(meters/second)  (3)        A longer stroke in a given time at a lower force can produce the        same amount of power as a shorter stroke in the same time at a        higher force, or vice versa. In practical applications of the        piston approach, there is a limit on the length of piston stroke        allowed. This is because practical devices such as a hydraulic        cylinder have a certain amount of stroke, and exceeding that        physical limit damages the cylinder. Also, in a given location,        the waves are normally in a known range of sizes during the        year. Thus, it would be economically impractical to provide        equipment that could stroke farther than would be caused by the        normally present waves. Prevention of damage by larger than        normal waves, such as storm waves (not illustrated), is by        pressure relief doors in the tube 10 above and below the pistons        12 range of motion. If a wave produces a pressure differential        (and resulting piston force) across the piston that is more than        a preselected valve, the doors are pushed open. This allows        water to bypass the piston, reducing its force and preventing        damage to the device.

A piston system will normally have provision for a certain physicalstroke range such as 1 meter, but could be longer or shorter. However,the force can be increased or decreased by simply making the unit andits piston larger or smaller. This is an important factor in the designof a practical system, and is based on the fact that fluid pressure doesnot depend on the size of the area it is acting upon. For example,assume that waves are expected to be present that provide an averagepressure differential of 2,000 Pascals (Pa) between the top and thebottom of the tube, as described above. A Pascal is a pressure of oneNewton per square meter. Also, assume that the waves have a period of 5seconds, that is, a wave will move from a peak to a trough in 2.5seconds. If the piston stroke is limited to 1 meter, and it is movingits full stroke, it will have an average velocity of 1/2.5=0.4 m/s. If1000 watts, or 1000 Nm/s, is desired from the system, then the averageforce must be (1000 Nm/s)/(0.4 m/s)=2,500 N. Since the pressuredifferential is 2,000 Pa or 2,000 N/m², the piston area must be (2,500N)/(2,000 N/m²)=1.25 m². This corresponds to a piston diameter of 1.26meters (D).

The mass of water in the tube and piston area moves along with thepiston. It must be accelerated in one direction, decelerated to a stop,accelerated in the other direction, decelerated to a stop, and so on.Therefore, some of the force produced by the pressure differential onthe piston must be used to accelerate the water mass. This can becalculated as force equals mass times acceleration, orF_(water)=m_(water)a. Some of this force is recoverable from thedecelerating water. However, having to accelerate and decelerate a largewater mass causes the optimum tube 10 length to be shorter than ½ thewaves length. This is because a longer tube captures a higher pressuredifferential than a shorter tube, but also contains more water. Theoptimum tube 10 length can be calculated for a specific wave length,wave height, and water depth using Bernoulli's Equation as previouslydiscussed.

By way of concise summary, characteristics of the moving piston approachare:

-   1. A tube long enough to create a significant varying pressure    differential between its top and bottom ends when placed in waves    with a range of wavelengths.-   2. A piston within the tube that causes the varying pressure from    the top of the tube to occur at the top surface of the piston, and    the relatively constant pressure from the bottom of the tube to    occur on the bottom surface of the piston.-   3. Because of 1. and 2. the piston is driven up and down with force    and velocity.-   4. A means, such as a hydraulic cylinder and motor, to convert the    reciprocating mechanical power of 3. into rotary mechanical power.-   5. An electric generator to convert the rotary mechanical power    of 4. into electrical power.-   6. The piston diameter can be larger or smaller than the tube    diameter, producing either a relatively high force low velocity    motion or a relatively low force high velocity motion.-   7. The sizes of the system tube and piston components affect the    amount of water mass enclosed within the system which affects the    amount of acceleration of the piston and water that can be achieved    from a given wave environment.

Additional requirements, discussed further hereinafter, are:

-   8. The system can utilize a fixed mooring to the sea bottom, or a    mooring that provides a floating unit to balance the piston forces    with a properly sized buoyant section.-   9. The system can utilize a mooring that combines a fixed buoyant    mooring and additional buoyancy to compensate for tidal variations    by moving the tube top up and down with the tide.

Various mooring arrangements are described hereinafter.

2. Description of turbine approach: The principles of operation of unitsthat use a water turbine instead of a moving piston for power extractionare very similar. The key difference is the need for a high water flowvelocity. As shown in Equation 3, force and velocity make equalcontributions to power output.Power_(watts)=Force_(Newtons)×Velocity_(meters/second)  (3)The moving piston approach must limit the stroke and hence the velocityfor practical reasons. Thus, the force is emphasized by providing alarge piston area. When using a turbine to extract power from flowingfluids a high velocity is desirable to overcome initial static frictionto insure that the turbine starts rotating, and to provide efficientoperation. The power available for capture from a cross sectional areaof fluid flow is given by:Power_(watts)=0.5×A×ρ×V ³  (4)

-   -   Where A is the cross sectional area of flow in m², ρ is the        density of the fluid (1000 kg/m³ for water), and V is the        velocity in meters per second.    -   A high average velocity is desirable to optimize power output.        The maximum thrust, or force, on a turbine in fluid flow is        given by:        Thrust_(Newtons)=(⅜)×A×ρ×V ²  (5)        Once the velocity profile has been determined for a certain tube        configuration and wave profile, the expected power output can be        calculated from Equation 4. Also, the necessary buoyancy volume        to balance the thrust force and maintain stationary position for        the tube can be calculated from Equation 5.

Mooring of the above-described embodiments is now described.

In the example shown in FIG. 2, a mooring attachment to the sea bottomis shown. The mooring attachment acts as a mechanical datum to resistthe upward and downward forces of the piston and keeps the tube fixed inplace. The mooring attachment must be strong enough to withstand thedownward forces produced by the unit, and heavy enough to resist theupward forces produced by the unit. It must be strongly attached to theocean bottom to resist the forces produced by storm waves.

In other sites the water may be too deep for practical bottom mounting.FIG. 5 shows an arrangement for mooring the inventive systems invirtually any depth of water because the length of its mooring chain 17is variable. The tube 10 is held in vertical position by buoyancy tanks50 attached to its outer perimeter. These buoyancy tanks aresufficiently buoyant to float the unit to the surface were it not heldby its mooring chain or cable. The tanks are buoyant enough to supportthe weight of the unit plus at least the maximum downward force exertedby the piston against the tube. This prevents the tube from moving lowerduring normal operation of the power producing tube. The mooring chainmust be at least strong enough to resist the net upward force of thebuoyancy tanks, plus the maximum upward force produced by the pistonagainst the tube. The anchor also must weigh at least as much as the netupward force of the buoyancy tanks plus the maximum upward forceproduced by the piston against the tube to prevent lifting of theanchor.

The fixed depth mooring arrangements shown in FIGS. 2 and 5 will allowtidal changes in water depth to affect power capturing performance. Innormal tides, for example 1 meter, the effect is small. A preferredmooring plan is to moor the unit at its planned depth below the surfaceat the midpoint of the tidal change. Then some times it will be deeperbelow the surface (high tide), and some times it will be closer to thesurface (low tide) than planned. Table 1 indicates that the energy levelat the top of a tube that is 1 meter below the mean surface of waveswith a 7 second period is 4% less than if it were 0.5 meters below themean surface. Thus a unit that was moored 1 meter below the surface atmid-tide in a 1 meter tidal environment would range from plus or minus0.5 meters from the planned depth during a day. A unit moored so shallowthat wave troughs expose the top of the unit suffers little or no lossin power output. Therefore, such a unit fixedly moored as discussedabove will produce approximately at its average planned level in anormal tidal environment. In areas with high tides, the unit ispreferably mounted lower in the water to prevent excessive exposureduring wave troughs. This will reduce the average power that the unitcan capture as can be estimated from Table 1. To meet a certain powergoal, a slightly larger unit is required than if the site had smallertidal changes. The simplicity of a fixed mooring arrangement generallyoutweighs the power loss in sites with range of depths and tides thatare not extreme.

A second mooring approach, shown in FIG. 6, combines a fixed bottommounting and a floating tube top. The fixed bottom mounting provides thesimplicity discussed above, and the float provides tidal compensation.In this case, the top portion of the tube 15 is flexible and can beextended upward by the buoyancy of a small float 62 when the tide ishigh and raises the mean water level. When the tide is low, the float 62follows the water level downward compressing and shortening the flexibletop tube section 15. The float maintains the top of the tube at arelatively fixed depth below the water surface, e.g., 1 meter. Theapparent change in the water height above the tube is approximately thesame whether the water is rising and falling above a fixed open tubetop, or is rising and falling above the tube extension. In thisarrangement, the large forces produced by the piston working in itspressure driven mode are countered by the fixed mooring buoyancy tanks50, while the added buoyancy tanks 62 only raise and lower the top ofthe tube.

A second embodiment of the invention, illustrated in FIGS. 7 through 9is now described.

FIGS. 7-9 show a hollow tube 110 having a closed top end 112 and an openbottom end 114. As previously described in the first embodiment, thetube 110 is in vertical, submerged orientation but, unlike the tube 10in the first embodiment, which is preferably fixed in place, the tube110 of the second embodiment is vertically movable relative to a fixedsupport. Such support can be a rigid structure mounted on the water bed,but, especially in deep water, is preferably a float 116 fixedly mooredto the water bed 118 by an anchor 120 and a chain or cable 128.

Most conveniently, the tube 110 encloses the float 116 and, because thetube is vertically elongated, the float 116 is similarly elongated.

The float 116 has a large buoyancy, and corresponds to a fixed structurerigidly mounted on the water bed but with the exception that somehorizontal displacement of the float can occur in response to horizontalwater movements. Such horizontal displacements of the float willgenerally occur at a slow rate and, essentially, the function of thefloat is to provide a definite position of the tube relative to thewater bed. In situations where large changes in the water level occur,means, generally known, are used for adjusting the distance between thefloat and the water bed for maintaining a fixed distance between thefloat and the water surface. However, and as explained in thedescription of the first embodiment, power generation is relativelyinsensitive to moderate water level changes and, typically, the float ispositioned for optimum performance at the average water level and notthereafter changed in position with water level changes.

The tube 110 is secured to the float 116 by means of a hydraulic pump122 of known type comprising a rigid casing 124 with a piston rod 126(for pumping fluids within the pump) extending entirely through andoutwardly from both ends of the casing 124. Herein, the pump casing 124is rigidly secured to the movable tube 110 by a spoke-like bracket 121(so as not to impede water movement within the tube 110). The upper endof the pump casing 124 is rigidly secured to the closed top end of thetube 110 but with one end 126 b of the piston rod 126 extending throughthe tube end. (Optionally, a navigation aide 127 is attached to the rodend 126 b and extends above the surface of the water.) The other end 126a of the piston rod 126 is rigidly secured to the float 116. The tube isneutrally buoyant and includes a hollow buoyancy chamber 125. Beingneutrally buoyant, the tube 110 vertically oscillates in response totube top-to-bottom pressure variations caused, as previously described,by passing waves. Vertical oscillations of the tube 110 relative to thefixed float 116 thus cause relative movements between the pump casing124 and the pump piston rod 126, the result being the generation ofalternating hydraulic pressures within the pump which can be used forpressure circulating a fluid through hoses 111 a for driving a hydraulicmotor-electrical generator 111.

Factors influencing the design of the overall system are similar tothose described in the description of the first embodiment. Therein, apiston within a stationary tube moves in response to passing waves.Herein, the closed (upper) end 112 of the tube 110 functions as a pistonmovable relative to a fixed support.

During operation, the tube upper end 112 remains submerged for allpassing waves within a range of wave sizes with which the system isdesigned to operate. For avoiding excessive forces due to extra largewaves, pressure relief valves are used, e.g., in the form of springbiased doors 130 shown in FIG. 10 at the top end 112 of the tube 110.Herein, four doors 130 are shown. If the pressure differential betweenthe water above the tube and the water inside the tube exceeds apreselected level, two of the doors open downwardly to equalize thepressure within and outside the tube 110. The other two doors 130 openupwardly to relieve internal excess pressures due to excessively deepwave troughs passing over (or beneath) the tube upper end 112. Thespring bias for the doors 130 can be obtained from weights or buoyantcompartments on the doors.

An advantage of the cable anchored arrangement shown in the figures isthat the unit is free to move horizontally due to wave action. Thisreduces the horizontal forces imposed on the mooring and reduces themass of the required mooring. Large horizontal movements tend to lowerthe tube upper end 112 relative to the water surface. This loweringtends to reduce the output power from the unit otherwise obtainable whenthe tube upper end 112 is optimally spaced beneath the water surface(previously described). However, as previously noted, changes in powerproduction with increased spacings of the tube end from the watersurface are rather gradual, and useful power production continues evenwith large horizontal leanings of the unit.

Relative horizontal movements between the float 116 and the tube 110 arepreferably avoided for avoiding damage of the mechanical couplingtherebetween. For such movements to occur, in response to lateralmovements of the tube 110, water must move within the tube 110 from sideto side of the float 116. Such water movements, and attendant relativelateral movements of the float 116 relative to the tube 110, areessentially prevented by the use of vertically elongated, radiallyextending fins 117 shown in FIGS. 8 and 9.

The above-described arrangement of a float 116 within a tube 110provides a self-contained unit which can be readily assembled on-shoreand transported for simple placement at an ocean site. In sucharrangement, the float 116 serves as a fixed support on which atransducer is fixedly mounted; the transducer, in turn, being connectedto and driven by the movable tube 110.

In an alternate arrangement, shown in FIG. 10, a transducer 222 (e.g., ahydraulic tube or the like) is fixedly mounted on the floor of the ocean(preferably by a mechanical coupling, e.g., a ball-socket joint 232,allowing pivoting of the transducer 222), with the movable piston rod226 of the transducer rigidly connected to the bottom end of a neutrallybuoyant tube 210 identical to the tube 110 shown in FIGS. 7-9 but notincluding an internal float. The tube 210 is connected to the piston rod226 (again, preferably by a pivoting coupling) by an anchoring link 228which can be an anchor chain or, preferably, a solid rod having a highmodulus of elasticity, i.e., low straining with applied stress.

In the very first patent (U.S. Pat. No. 4,404,490) issued to the ownerof the present invention, reference is made to “cancellation effects”,i.e., the energy robbing effect when the dimensions of the wave energycollector are a significant fraction of the wave length of the surfacewaves (the subject matter of such patent being incorporated herein byreference). Herein, for example, top to bottom pressure variationsacross the lengths of the various herein disclosed tubes occur inresponse to passing waves. If, for example, the diameter of the tubeswere equal to the wave lengths of the passing waves, the pressureincreases caused by the wave crests overlying the tube top ends would becancelled by the simultaneous presence of the overpassing wave troughs.Thus, no vertical oscillations of the tubes would occur. Ocean waves,however, tend to be quite large and, for practical reasons, the diameterof the tubes are so small in comparison with the wave lengths thatcancellation effects can be ignored—provided that the tube diameters arenot in excess of a relatively small proportion of the wave length, e.g.,10%.

Any such cancellation effect occurs in directions parallel to thedirections of movements of the waves. No cancellations occur indirections normal to the wave directions, hence quite large area tubescan be used of rectangular cross-section provided the axis of greaterlength (in excess of 10% of the wave length) is maintained perpendicularto the wave direction.

Within the embodiment shown in FIG. 10, the transducer 222 disposedbelow and outside the tube 210, a hollow space within the tube 210 forcontaining the transducer 222 and a float 116 (as in FIG. 7) is notrequired, and the tube 210 need not be hollow and need not have an openbottom end. The only requirements for the tube 220, in accordance withthe present invention, are that it is similar to the tube 110 in that ithas the same outside dimensions (for use with the same wave environment)and has a closed top end serving as a piston responding to surface wavepressure variations. The tube 220, similarly as the tube 110, must beneutrally buoyant, for vertical oscillations in response to top tobottom pressure variations caused by overpassing waves, but the tube canbe hollow or solid to any extent as may be desired.

In all the inventive embodiments disclosed herein, the energy capturingmember, e.g., the movable cylinder 110 shown in FIG. 7, receives energyfrom the passing surface waves by virtue of water pressure variations.Specifically, a float bobbing up and down on the water surface, such ascommonly disclosed in the prior art, is not used. This difference fromthe prior art can be expressed by such statements that the energycapturing elements, e.g., the movable cylinder 110, are structurallyseparated from the surface of the water (i.e., not connected to asurface float) and/or that such energy capturing elements are mounted ona support disposed in essentially fixed position relative to the meanwater level. Such support can be anchored to the floor of the body ofwater or, alternatively, can comprise a float of such size as to berelatively vertically stationary in response to passing waves.

Further, while FIG. 7 shows a mast 126 projecting upwardly above thewater surface, the mast is not mounted on the movable cylinder 110,hence does not structurally interconnect the cylinder 110 to the watersurface. However, even if the mast 126 were mounted directly on thecylinder 110 for movements therewith, the mast 126 does not, as hereindefined, structurally interconnect the cylinder to the water surfacebecause the mast captures no energy, as does a float, from passingsurface waves.

As described, energy capture is by virtue of water pressure variations.In co-pending application Ser. No. 10/080,181, filed Feb. 20, 2002 (thesubject matter of which is incorporated herein by reference), there isdisclosed an arrangement for capturing energy present in tiltingmovements of the vertically extending apparatus caused by movements ofthe surrounding water. Such arrangement can be used with the presentinvention.

1. A method for capturing energy from preselected surface waves on abody of water, the waves varying from a maximum to a minimum wavelengthand having a maximum amplitude above and below a mean water levelpresent during the passage of said waves, the method comprisingdisposing in said body of water an elongated cylinder in vertical andcompletely submerged orientation relative to said mean water level, thecylinder having a top end structurally separated from the surface of thewater and disposed at a first depth approximately equal to said maximumamplitude, and the cylinder having a bottom end disposed at a seconddepth where the energy level associated with waves at said maximumwavelength is a small percentage of the energy associated with saidmaximum wavelength waves at said mean water level, and preventing waterflow through the cylinder in response to cylinder top to bottom waterpressure differentials caused by said passing waves for causing energycapturing movements of said cylinder.
 2. A method according to claim 1wherein the said second depth is equal to about 50% of said maximumwavelength waves.
 3. A method according to claim 1 including mountingsaid cylinder for limited vertical movements on a support in essentiallyfixed position relative to said mean water level.
 4. A method accordingto claim 3 wherein said stationary support is a float, and including thesteps of anchoring said float beneath the surface of the body of waterand mounting said cylinder in enclosing relationship with said support.5. A method of capturing energy from surface waves on a body of watercomprising: submerging an elongated member within said water body invertical orientation with a closed end of said member up; mechanicallysupporting said member in vertical movable relation with, and solely by,a support disposed in essentially fixed position relative to the meanwater level of said body of water such that a principal mechanism fortransferring energy from said surface waves to said member comprisesvariable water pressure differentials between opposite ends of saidmember.
 6. A method according to claim 5 including disposing said memberwithin said body of water in relation to the mean water level of saidbody such that variations in water pressure, caused by overpassingwaves, at the top of said member are at least 50% larger than the waterpressure variations caused by said waves at the bottom of said member.7. A method according to claim 5 including the step of anchoring saidsupport to the floor of said body of water.
 8. A method according toclaim 7 wherein said support is a float, and said elongated member ishollow, and including the step of mounting said elongated member inenclosing relation with said float.